1. Field of the Invention
This invention relates to a developing apparatus in electrophotographic machines such as copying machines.
2. Description of the Prior Art
Conventional electrophotographic machines such as copying machines have, as shown in FIG. 4, a developing apparatus 1 and a toner supply box 2 from which toner is supplied to the developing apparatus 1. In the developing apparatus 1, agitation rollers 3 and 4 admix the toner with a carrier such as iron powder or the like and agitate the mixture, resulting in a developer. The developer is fed to a magnet roller 5 by the agitation rollers 3 and 4. The magnet roller 5 is composed of a sleeve 51 and a plurality of magnets 52 with two main polarities N.sub.1 and N.sub.2 and three supplementary polarities S.sub.1, S.sub.2 and S.sub.3. The polarity S.sub.1 is positioned between the N.sub.1 and N.sub.2 and the polarity S.sub.2 is positioned between the S.sub.3 and N.sub.1. By means of these polarities N.sub.1 -N.sub.2 and S.sub.1 -S.sub.3, the developer attaches to the sleeve 51 that is disposed around the magnets 52, and is sent to a photosensitive drum 6 while the sleeve 51 rotates around the magnet 52 by a driving means (not shown). Since the carrier bearing the toner is made of magnetic material such as iron powder or the like, the carrier adheres to the surface of the sleeve 51 and forms a magnetic brush thereon in the direction of magnetic lines of force of the magnets 52. The direction and the density of the magnetic lines of the developer on the sleeve 51 vary as the sleeve 51 turns. Toner constituting the magnetic brush is supplied to the photosensitive drum 6 by the rotation of the sleeve 51.
Above the magnetic roller 5, a doctor blade 7 is provided as a means to regulate the amount of developer to be carried toward the photosensitive drum 6. There is a developer cover 8 downstream of the doctor blade 7 in the direction of conveyance of the developer. The tip of this doctor blade 7 is near the surface of the sleeve 51 of the magnetic roller 5 so as to regulate the amount of developer that is attached to the sleeve 51, so that a suitable amount of developer can be conveyed toward the photosensitive drum 6.
However, in the conventional developing apparatus with the above-mentioned construction, toner is blown into the direction of the developer cover 8 from the gap between the tip of the doctor blade 7 and the surface of the sleeve 51 because there is a pressure depression in the area of the downstream side of the doctor blade 7 in the direction of conveyance of the developer (i.e., in the area of the developer cover side). The toner, being a movable powder, is not held in a space between the developer cover 8 and the sleeve 51 of the magnetic roller 5, but is blown towards the outside of this developing apparatus 1. Thus, an appropriate amount of toner cannot be supplied to the photosensitive drum 6, so that an electrostatic latent image that has been formed on the photosensitive drum 6 cannot be uniformly developed by the toner.